1. Field of the Invention
The present invention relates to a method of inspecting a component part of an apparatus included in a steam turbine system.
2. Description of the Related Art
A steam turbine always has a possibility that damage occurs due to the aged degradation and erosion of the component parts. In order to maintain the rated performance and efficiency of the turbine, the turbine is inspected periodically to find damaged parts before the parts are damaged seriously.
Principal component parts to be inspected periodically of the steam turbine include moving blades, nozzles, a rotor, casings of high and medium-pressure stage turbines, valves, and steam pipes. Most defects that occur in those principal parts are cracks and fissures due to fatigue or creep, and thickness reduction and breakage due to erosion. Although it is possible that cracks are formed in portions of an outer surface of a turbine casing on which stress is concentrated, most defects occur on surfaces of the component parts exposed to high temperature and steam and thus subjected to quality degradation, creep, thermal fatigue and erosion.
In inspecting a turbine, the upper half casing is removed, and radial and axial clearances between the rotating and stationary component parts are measured. Then the rotor is hoisted, the rotor, the moving blades, the nozzles and the turbine casing are visually inspected for defects, erosion and cracks, and non-destructive inspection such as PT, MT or UT is carried out. Then it is judged whether the component parts should be repaired or replaced, and they are repaired or replaced if necessary. Then, the rotor is returned into the turbine casing, clearances are measured and adjusted, and the upper half casing is set in place.
When defects (e.g., cracks) are found by inspection in component parts which permit of no defects therein, such as rotating parts, it is usual to take measures immediately to eliminate the defects.
When defects are found by visual inspection in a non-rotating component part, such as a turbine casing or a valve casing, the risk of possible troubles is determined based on data on the degree of degradation of the material and stress distributions in the component part. The degree of degradation of the material is examined through replication, hardness measurement or electrochemical test. The stress distribution is estimated by minute analysis using a FEM (finite element method).
In detail, when a defect is found by inspection and then the size and shape of the defect are measured, consumed life (i.e., creep damage and/or fatigue damage accumulated in the damaged parts) of the damaged part is calculated based on the record of operation of the damaged part and the degree of degradation of the material examined through the non-destructive test. Based on the calculation result, remaining life, which is dependent on the possibility of formation and development of cracks, of the damaged part is estimated by using the factors including: temperature/stress probability distribution determined by the finite element method based on the shape and operating condition of the member; future mode of operation; and the aged degradation of the material. Then, the condition of the damaged part is determined based on the estimated remaining life, the risk level of the defect is determined and countermeasures to eliminate the defect are planed.
However, if the turbine is disassembled for inspection, a considerably long inspection time is necessary, that is, it takes about thirty days from shutdown of the turbine to the operation of the turbine at the rated temperature after restart of the turbine.
Moreover, estimation of stress distribution for the determination of the risk level of the defect takes time, and the estimated stress distribution is greatly dependent on a temperature boundary condition used by analysis by a FEM. Since most plants do not have sufficient data, and hence the accuracy of analysis by a FEM is not necessarily satisfactory. Consequently, the accuracy of determination of the risk level of a defect, such as a crack, found by visual inspection is limited.
In order to shorten the inspection time, a non-disassembling inspection method has been proposed. The method does not open the casing and inserts a fiberscope or a CCD camera through the flanged end of a steam pipe into the turbine casing. This inspection method omits a step of opening the casing, and a step of adjusting the clearances between the casing and blades, which is inevitable when the turbine casing is opened. In this method, however, the fiberscope or the CCD camera is inserted into the turbine casing for inspection after the temperature of the interior of the turbine casing has dropped near to an ordinary temperature. Thus, this method also takes a considerable long time, and also has the problem on unsatisfactory accuracy of the risk level determination.